Well, as you stated the IP's are different but all within the same range.

this means they are all part of the same LAN.

That they differ could be due to the fact that one is for instance from the ISP's proxy, another from the second webproxy, etc.
It could be that the ISP performs a ping or similar actions...

If the IP resembles to your IP, for instance if you have the following IP:
(fiction)
216.210.10.11
and an IP like
216.210.10.15
tries to connect to your box that could be another user from the same ISP.